1. Field of the Invention
Exemplary embodiments of the present invention relate to a light emitting device and a method of fabricating the same, and more particularly, to a light emitting device having a plurality of non-polar light emitting cells and a method of fabricating the same.
2. Discussion of the Background
Gallium Nitride (GaN)-based light emitting diodes (LEDs) are widely used for display and backlights. Further, LEDs have less electric power consumption and a longer lifespan as compared with conventional light bulbs or fluorescent lamps. Thus, LEDs have substituted conventional incandescent bulbs and fluorescent lamps. LED application areas have been expanded to the use thereof for general illumination.
In general, a GaN-based nitride semiconductor is grown on a heterogeneous substrate, such as a sapphire or silicon carbide substrate. The nitride semiconductor is mainly grown on a c-plane of such a substrate and has piezoelectric properties. A strong polarization electric field is generated in an active region of a multiple quantum well structure due to the piezoelectric properties. Therefore, it is difficult to increase the thickness of a light emitting layer. Thus, LEDs luminous power may not be significantly improved due to a decrease in light emitting recombination rate.
To prevent the generation of such a polarization electric field, a technique has recently been studied in which an a-plane nitride semiconductor is grown by machining GaN crystals grown on a c-plane sapphire substrate into a GaN substrate having a crystal face except the c-plane (e.g., an a-plane or m-plane), and using the GaN substrate as a growth substrate of a nitride semiconductor, or using an m-plane silicon carbide substrate or r-plane sapphire substrate as the growth substrate. The nitride semiconductor with the a-plane or m-plane has non-polar or semi-polar properties. Accordingly, it is expected that the nitride semiconductor has improved luminous power as compared to a polar LED having a polarization electric field.
LEDs generally emit light by forward current and require supply of DC current. Attempts have been made to develop a technique wherein a plurality of light emitting cells are driven by an AC power source by connecting the plurality of light emitting cells in reverse is parallel or using a bridge rectifier, and LEDs fabricated by this technique have been commercialized. Further, an LED has been developed which can emit high-output and high-efficiency light by a high-voltage DC power source by forming a plurality of light emitting cells on a single substrate and connecting them in series.
To use a LED connected to a high-voltage AC or DC power source using a plurality of light emitting cells, the plurality of light emitting cells are electrically separated from one another and are connected through wires. Since a conventional sapphire substrate is an insulative substrate, the plurality of light emitting cells can be electrically isolated from one another when using a nitride semiconductor grown on the sapphire substrate. However, since a GaN substrate generally has characteristics of an n-type semiconductor, when a plurality of light emitting cells are fabricated using non-polar or semi-polar nitride semiconductor layers grown on the GaN substrate, there is a problem in that the light emitting cells may be electrically connected by the GaN substrate.
To solve such a problem, nitride semiconductor layers may be grown on a GaN substrate, and the GaN substrate may then be separated from the nitride semiconductor layers. Conventionally, nitride semiconductor layers are grown on a sapphire substrate, and the sapphire substrate is then separated from the nitride semiconductor layers using a laser lift-off process. However, since the GaN substrate and the nitride semiconductor layers grown thereon have similar physical and chemical properties, it is difficult to separate the nitride semiconductor layers from the GaN substrate.